Blood pump

ABSTRACT

A blood pump comprises a pump casing having a blood flow inlet and a blood flow outlet connected by a passage, and an impeller arranged in said pump casing so as to be rotatable about an axis of rotation. The impeller is provided with blades sized and shaped for conveying blood along the passage from the blood flow inlet to the blood flow outlet, and is rotatably supported in the pump casing by a first bearing at a first axial end of the impeller and a second bearing axially spaced apart from the first bearing. The first bearing comprises a projection extending along the axis of rotation and connected to one of the impeller and the pump casing and a cavity in the other one of the impeller and the pump casing, the projection comprising an enlarged portion that engages the cavity such that the first bearing and the second bearing are arranged to bear axial forces in the same axial direction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a United States National Stage filing under 35U.S.C. § 371 of International Application No. PCT/EP2016/055646, filedMar. 16, 2016, which claims the benefit of European Patent ApplicationNo. 15159677.2, filed Mar. 18, 2015, the contents of all of which areincorporated by reference herein in their entirety. InternationalApplication No. PCT/EP2016/055646 was published under PCT Article 21(2)in English.

BACKGROUND

This invention relates to blood pumps to be implanted in a patient forsupporting the patient's heart. In particular, the blood pump may beused as a “bridge to recovery” device, whereby the blood pumptemporarily supports the patient's heart until it has sufficientlyrecovered.

Blood pumps of different types are known, such as axial blood pumps,centrifugal blood pumps or mixed type blood pumps, where the blood flowis caused by both axial and radial forces. Blood pumps may be insertedinto a patient's vessel such as the aorta by means of a catheter, or maybe placed in the thoracic cavity. A blood pump typically comprises apump casing having a blood flow inlet and a blood flow outlet connectedby a passage. In order to cause a blood flow along the passage from theblood flow inlet to the blood flow outlet an impeller is rotatablysupported within the pump casing, with the impeller being provided withblades for conveying blood.

The impeller is supported within the pump casing by means of at leastone bearing, which may be of different types depending on the intendeduse of the blood pump, for instance whether the blood pump is intendedonly for short term use (some hours or some days) or long term use(weeks or years). A variety of bearings are known, such as contact-typebearings and non-contact bearings. In non-contact bearings the bearingsurfaces do not contact each other, e.g. in magnetic bearings, in whichthe bearing surface “levitate” due to repelling magnetic forces.Generally, contact-type bearings may include all types of bearings, inwhich the bearing surfaces may contact at least partially duringoperation of the pump at any time (i.e. always or intermittently), e.g.in slide bearings, pivot bearings, hydrodynamic bearings, hydrostaticbearings, ball bearings etc. or any combination thereof. In particular,contact-type bearings may be “blood immersed bearings”, where thebearing surfaces have blood contact. Contact-type bearings, such aspivot bearings, may heat up during use and are subject to mechanicalwear. Mechanical wear may be increased by a magnetic coupling betweenthe electric motor of the pump and the impeller for driving theimpeller. When the contact-type bearing and the magnetic coupling aredisposed at the same axial end of the impeller, mechanical wear of thebearing may increase because the magnetic coupling attracts theimpeller, thereby increasing the contact pressure on the bearingsurfaces. This may lead to a high load (e.g. 10 Newton) on a smallbearing surface (e.g. in a pivot bearing in catheter pump e.g. in therange of 1 mm in diameter).

SUMMARY OF THE INVENTION

It is a primary object of the present invention to provide a blood pumphaving an impeller that is supported in a pump casing by means of abearing, wherein mechanical wear of the bearing can be reduced. Inparticular, it is another object of the present invention to relieve abearing in a blood pump from excessive loading.

The primary object is achieved according to the present invention by ablood pump with the features of independent claim 1. Preferredembodiments and further developments of the invention are specified inthe claims dependent thereon.

Like known blood pumps, the blood pump according to the inventioncomprises a pump casing having a blood flow inlet and a blood flowoutlet connected by a passage. An impeller or rotor is arranged in saidpump casing so as to be rotatable about an axis of rotation, which maybe the longitudinal axis of the impeller, with the impeller beingprovided with blades sized and shaped for conveying blood along thepassage from the blood flow inlet to the blood flow outlet. The impelleris rotatably supported in the pump casing by a first bearing at a firstaxial end of the impeller and a second bearing axially spaced apart fromthe first bearing.

According to the invention, the first bearing comprises a projectionextending along the axis of rotation and connected to one of theimpeller and the pump casing and a cavity in the other one of theimpeller and the pump casing. The term “cavity” may comprise any kind ofcavity, hollow space, recess, opening, or bore. The projection comprisesan enlarged portion that engages the cavity such that the first andsecond bearings are arranged to bear axial forces in the same axialdirection. For instance, the enlarged portion may be supported by orenclosed in the cavity. In other words, the first bearing relieves thesecond bearing by carrying at least part of the axial force that acts onthe bearing surfaces of the second bearing. Since both bearings supportthe impeller in the same “bearing direction”, the load on each of thebearings can be reduced. They “share” the load. This is achieved byproviding simple mechanical means in the form of a projection with anenlarged portion and a corresponding cavity such that the impeller is toa certain extent “suspended” by the first bearing. Preferably, theprojection and the enlarged portion are comprised in the impeller, withthe cavity being static and part of the pump casing. The cavity may bedisposed in a supporting structure of the pump casing. It will beappreciated, however, that the arrangement may be vice versa withoutaffecting the functionality.

Preferably, the cavity corresponds in size and shape to the enlargedportion. This may improve the bearing characteristics because adisplacement of the impeller in an axial direction or radial directionor both can be reduced or avoided if the cavity and the enlarged portionare adapted to each other in size and shape. In particular, the firstbearing can be arranged to bear axial forces in two opposed axialdirections. However, it is not necessary that the cavity corresponds insize and shape to the enlarged portion as long as the first bearing canbear axial forces in the same axial direction as the second bearing. Forinstance, in an opposite axial direction, i.e. the direction away fromthe second bearing, the cavity may be open or may at least provideenough space such that a movement of the impeller in this direction ispossible.

Preferably, the enlarged portion is at least partially spherical inshape. For example, the enlarged portion may be a spherical cap.Accordingly, the cavity may be at least partially spherical as well.However, the enlarged portion may have any shape that is suitable forachieving the inventive concept. In particular, any shape that isrotationally symmetrical and has a ledge to bear axial forces may besuitable for the first bearing. The enlarged portion may be e.g.cylindrical or conical. The enlarged portion may be snap fitted into thecavity, which is a simple way to mount the first bearing. The materialof the enlarged portion or the cavity or both may have sufficientresilient properties to allow a snap fit connection. In anotherembodiment the cavity or the enlarged portion may be formed by moldingover a preexisting part that is made for example from a metallic orceramic material or alike and allow the shrinkage of the molded plasticto create a dynamic joint.

In an embodiment, the second bearing may be a contact-type bearingcomprising a bearing surface of the impeller facing a bearing surface ofthe pump casing, preferably a pivot bearing. A pivot bearing allows forrotational movement as well as pivoting movement to some degree.

The blood pump may further comprise a shaft extending along androtatable about the axis of rotation and having the impeller mountedthereon, the shaft having a first end portion forming part of the firstbearing and a second end portion forming part of the second bearing. Inparticular, the first end portion of the shaft may comprise the enlargedportion, such as a spherical cap. The shaft may have an outer diameterthat is substantially equal to an outer diameter of the enlargedportion, with the projection forming a neck arranged between the shaftand the enlarged portion. This allows a compact arrangement and isadvantageous for the snap fit connection. The second end portion of theshaft may comprise a bearing surface of the second bearing, said bearingsurface being concave, e.g. spherical, for instance forming part of apivot bearing. It will be appreciated that the shaft may be separatelyformed or integrally formed with the impeller. It will be furtherappreciated that parts of the shaft are in direct blood contact forpurpose of improved heat transfer to the surrounding blood. The shaft ispreferably made of a heat conducting material (metal, silicon carbide,or similar material) so the heat generated within the portion of thebearing can be well transferred to the surrounding blood in order tolimit the temperature to 55° C. or less.

In an embodiment, a wall of the cavity may comprise at least twosections separated by a gap, for example two, three or four sections.The gap may be in fluid connection with the passage to allow blood toenter the cavity. This enables the first bearing to be washed out byblood flowing through the blood pump or by any other rinsing fluid.

In an embodiment, the enlarged portion of the first bearing may compriseat least one magnet, and the pump casing, preferably the cavity, mayalso comprise at least one magnet. The magnets may be arranged such thata repelling magnetic force pointing in an axial direction away from thesecond bearing is caused. This arrangement allows for relieving thesecond bearing because the magnetic force of the first bearing pulls theimpeller in a direction away from the second bearing. Apart from that,due to the design of the first bearing as a magnetic bearing, mechanicalwear of the first bearing in an axial direction can be avoided.

In an embodiment, at least one of the bearing surfaces of at least oneof the first and second bearings may be supported by at least onespring, wherein the at least one spring is arranged to bear axial forcesin an axial direction from the second bearing towards the first bearing.Specifically, the second bearing can be relieved by providing at leastone spring that receives and dampens part of the load that acts on thebearing surfaces of the second bearing. At least one spring may bedisposed on a side of the second bearing facing away from the firstbearing. For instance, at least one spring may be disposed in a portionof the casing supporting a static bearing surface of the second bearing.The spring force may be chosen to be less than the load on the secondbearing. In this case, the load on the second bearing is limited to theamount of the spring force, while the remainder of the load is supportedby the first bearing. Alternatively or in addition, the first bearingmay be spring supported by at least one spring, in particular theportion of the pump casing comprising the cavity, whereby the springcauses a force in an axial direction away from the second bearing Itwill appreciated that the spring can be composed of a metallic materialor may be formed out of a polymeric material for example in the shape ofa O-ring or similar shape that serves the same function as a coilmetallic spring. In an embodiment, at least one of the bearing surfacesof second bearing may be supported by a flexible structure that may havea known force when flexed.

Further means for relieving the second bearing can be provided. Forinstance, repelling magnets could be provided in or near, e.g. around,the bearing surfaces of the second bearing. In order to reduce oreliminate axial forces caused by the attracting magnetic forces thattransmit rotation to the impeller, the magnetic drive arrangement may bedisposed in a radial direction, where the magnetic drive iscircumferentially arranged around the impeller. Likewise, in anarrangement in which the magnetic drive is disposed in an axialdirection with respect to the impeller, the magnetic flux may bedeflected such that it acts on the impeller in a radial direction.Further, alternatively or additionally, a supporting bearing may beprovided that carries part of the load of the second bearing. Thesupporting bearing may be a ball bearing that comprises a plurality ofballs running between a surface of the impeller and a surface of thehousing, preferably in respectively aligned grooves, surrounding thesecond bearing.

It will be appreciated that according to the invention two mechanicalbearing are employed to center the impeller. The spatial distribution ofboth bearings along the axis of rotation allows for a stiffbearing/impeller arrangement. The later allows for pulsatile pumpoperation where the impeller may transfer from below critical speedswith every heart cycle. Due to the stiff arrangement provided by the twomechanical bearings, vibration of the impeller during critical speedscan be reduced. The pulsatile and synchronized pump operation with theheart is considered to be highly advantageous to achieve heart recovery.In addition it will be appreciated that mechanical bearings are employedbecause of their small size, which is mandated by any small sized pumpintended to be placed inside a patient's vessel.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofpreferred embodiments, will be better understood when read inconjunction with the appended drawings. For the purpose of illustratingthe present disclosure, reference is made to the drawings. The scope ofthe disclosure is not limited, however, to the specific embodimentsdisclosed in the drawings. In the drawings:

FIG. 1 shows a cross sectional view of a blood pump according to theinvention.

FIG. 2 shows a cross sectional view of a blood pump according to anotherembodiment.

FIG. 3 shows a cross sectional view of a blood pump according to stillanother embodiment.

FIGS. 4A-4D show different embodiments of the first bearing.

FIG. 5 shows a cross sectional view of a blood pump according to anembodiment, in which the blood pump is designed as a catheter pump.

FIG. 6 shows a cross sectional view of a first shaft of the blood pump.

FIG. 7 shows a cross sectional view of a second shaft of the blood pump.

FIG. 8 shows a cross sectional view of a supporting structure for thefirst shaft.

FIGS. 9A-9C show cross sectional views of a supporting structure for thefirst shaft according to other embodiments.

FIG. 10 shows a cross sectional view of a blood pump according toanother embodiment.

FIG. 11 shows a cross sectional view of a supporting structure for thefirst shaft in connection with the embodiment of FIG. 10.

FIG. 12 shows a cross sectional view of a blood pump according toanother embodiment.

FIG. 13 shows a cross sectional view of a blood pump according toanother embodiment.

FIG. 14 shows a cross sectional view of a blood pump according toanother embodiment.

FIG. 15 shows a cross sectional view of a blood pump according toanother embodiment.

DETAILED DESCRIPTION

Referring to FIG. 1, a cross sectional view of a blood pump 1 isillustrated. The blood pump 1 is designed for extracorporeal,extracardiac or extraluminal use and comprises a pump casing 2 with ablood flow inlet 5 and a blood flow outlet 6. During operation, the pumpcasing 2 is placed outside a patient's body and the blood flow inlet 5and the blood flow outlet 6 are connected to respective connectors (inparticular inflow from the heart and outflow to the aorta). The blood isconveyed along a passage 7 connecting the blood flow inlet 5 and theblood flow outlet 6. An impeller 3 having a shaft 14 is provided forconveying blood along the passage 7 and is rotatably mounted about anaxis of rotation 9 within the pump casing 2 by means of a first bearing10 and a second bearing 20. The axis of rotation 9 is preferably thelongitudinal axis of the impeller 3. Both bearings 10, 20 arecontact-type bearings as will be described in more detail below. Thesecond bearing 20 is a pivot bearing having spherical bearing surfacesthat allow for rotational movement as well as pivoting movement to somedegree. The first bearing 10 is disposed in a supporting member 15 tostabilize the rotation of the impeller 3, the supporting member 15having at least one opening 16 for the blood flow. Blades 4 are providedon the impeller 3 for conveying blood once the impeller 3 rotates.Rotation of the impeller 3 is caused by an electric motor 8 magneticallycoupled to an end portion 37 of the impeller 3. Other suitable drivingmechanisms are possible as will be appreciated by a person skilled inthe art. The illustrated blood pump 1 is a mixed-type blood pump,wherein the major direction of flow is axial. It will be appreciatedthat the blood pump 1 could also be a purely axial blood pump, dependingon the arrangement of the impeller 3, in particular the blades 4.

The impeller 3 comprises a portion 33 that extends radially outwards.The portion 33 can be denoted as a yoke, flange portion or end portion.At least one wash out channel 30, preferably two or more, such as three,four, five or six wash out channels 30, only one of which is shown inFIG. 1, extends through the impeller 3, in particular through theportion 33, so as to allow for washing out or rinsing a clearance 31between the impeller 3 and a static part of the blood pump 1, inparticular the pump casing 2 or the motor 8, which may be regarded asassociated with the pump casing 2. The at least one wash out channel 30may also extend at least partially into the main portion of the impeller3 beyond the portion 33. The wash out channel 30 has a first opening 34and a second opening 35. The first opening 34 forms a fluid connectionbetween the passage 7 and the wash out channel 30, while the secondopening 35 is in fluid connection with the clearance 31. In particular,the second opening 35 is in fluid connection with a central bore orcentral opening 32 of the portion 33 accommodating the second bearing20, such that the second bearing 20 can be washed out and cooled.

The second bearing 20 comprises a first bearing surface 23 disposed at asecond end portion 24 of the first shaft 14 (see FIG. 6) and a secondbearing surface 22 at an end portion of a second shaft 21, in particularin a recess at the center of a second shaft 21 (see FIG. 7). Bothbearings surfaces 22, 23 are preferably spherical. Due to the magneticcoupling between the electric motor 8 and the end portion 37, theimpeller 3 is attracted towards the motor 8 which increases the pressurebetween the bearing surfaces 22, 23 of the second bearing 20. In orderto relieve the second bearing 20, the first bearing 10 is arranged atthe opposed axial end portion 19 of the first shaft 14 and is arrangedto bear axial loads in the same axial direction as the second bearing20.

The first bearing 10 includes an enlarged portion 12 that engages acavity 13 in the pump casing 2, in particular in the supportingstructure 15, which may be regarded as part of the pump casing 2. Inparticular, the enlarged portion 12 may be supported by, enclosed in, orentrapped in the cavity 13. The enlarged portion 12 may be snap fittedinto the cavity 13 or otherwise mounted. More specifically, the firstbearing 10 includes a protrusion 11 extending axially at the first endportion 19 of the shaft 14 including the enlarged portion 12. In theembodiments shown, the enlarged portion 12 is formed as a cap that ispartially spherical in shape. However, any rotationally symmetric shapemay be chosen for the enlarged portion 12 that is suitable for bearingaxial loads in the same direction as the second bearing 20. Theprojection 11 forms a neck having a smaller diameter than the enlargedportion 12. As shown in FIG. 6, the enlarged portion 12 has the samediameter as the shaft 14. The diameters of the enlarged portion 12 andthe shaft 14 could be different. In particular, the diameter of theenlarged portion 12 could be smaller or greater than the diameter of theshaft 14. The neck also may be omitted. The shaft 14 may also beintegrally formed with the impeller 3.

In the embodiment of FIG. 1, the cavity 13 substantially corresponds insize and shape to the enlarged portion 12. Therefore, the first bearing10 does not only bear loads in the same axial direction as the secondbearing 20 but supports the impeller 3 in both axial directions. In theembodiment of FIG. 2, the cavity 13′ is open to a side that faces awayfrom the second bearing 20. However, the supporting structure 15 issized and shaped to correspond to the size and shape of the neck 11,such that the impeller 3 is supported in both axial as well as radialdirections. It will be appreciated that the supporting structure 15 maybe smaller to allow some axial movement in a direction away from thesecond bearing 20. In the embodiment of FIG. 3, the impeller 3 may movein an axial direction away from the second bearing 20, which may occurdue to the rotating movement of the impeller 3 in the blood flow. Inthis embodiment, the cavity 13″ is axially enlarged in a direction awayfrom the second bearing 20.

FIGS. 4A-4D show different embodiments of the first bearing 10, inparticular the enlarged portion 12 and the cavity 13. In FIG. 4A theenlarged portion is substantially spherical similar to the enlargedportion shown in FIGS. 1 to 3. The section that contacts the cavity 13in the supporting structure 15 may have a different diameter than theremaining section of the enlarged portion 12 and may be convex.Alternatively, this section may be concave. In the embodiment of FIG. 4Bthe enlarged portion 12 is conical or diamond shaped. FIGS. 4C and 4Dshow similar embodiments, wherein the enlarged portion 12 has a conicalor tapered section. This facilitates assembly of the bearing 10. In FIG.4C, the section of the enlarged portion 12 that contacts the cavity 3 inthe supporting structure 15 is spherical and convex, whereas it isconcave in FIG. 4D. It will be appreciated that any rotationallysymmetrical shape may be used for the enlarged portion 12.

Referring now to FIG. 5, an embodiment is shown that is similar to theaforementioned embodiment of FIGS. 1 to 3, in particular to that of FIG.2, with the difference that it is designed as a catheter pump 1′. Theblood flow inlet 5′ is at the end of a flexible cannula 50 which isplaced through a heart valve, such as the aortic valve, during use,while the blood flow outlet 6′ is placed in a side of the pump casing 2′and is placed in a heart vessel, such as the aorta. The blood pump V isconnected to a catheter 51, and an electric wire 52 extends through thecatheter 51 for driving the pump 1′. Both blood pumps 1 and V functionin the same way. It will be appreciated that all features described areapplicable for both extracorporeal pumps and catheter pumps.

Referring now to FIG. 8, a cross section through the supportingstructure 15 is shown, including openings 16 for allowing blood to flowthrough the supporting structure 15. The supporting structure 15 maycontribute one or more struts. In the embodiments of FIGS. 9A-9C, thewall of the cavity 13 comprises segments or sections 17 separated bygaps 18. The gaps 18 allow blood to flow into the cavity 13 to wash outthe cavity 13, in particular for cooling the first bearing 10. Thesections 17 may be denoted as stator blades that support the rotatingpart of the first bearing. In FIGS. 8 and 9A, the supporting structure15 is shown having three struts and three openings 16. The supportingstructure 15 may have fewer or more struts and openings, such as one(FIG. 9B), two (FIG. 9C), four, five, six or more.

An embodiment of a blood pump 1 that is substantially similar to theaforementioned embodiments is shown in FIG. 10. In this embodiment,however, the first bearing 10″′ is designed as a magnetic bearinginstead of a contact-type bearing. The enlarged portion 12′ comprises atleast one magnet 40 that are arranged to cause a repelling magneticforce against magnets 41 that are arranged in the supporting structure15. The enlarged portion 12′ is disposed on a projection 11′ thatengages a cavity 13″′. The repelling magnetic force aids in relievingthe second bearing 20. In FIG. 11 a cross-sectional view through anembodiment of a blood pump in which the first bearing 10 is designed asa magnetic bearing is depicted. Similar to the aforementionedembodiments, the supporting structure comprises three struts 15, whereina wall of the cavity 13″′ is divided into three segments 17′ separatedby gaps 18′. In the wall segments 17′, magnets 41 are disposed that acton respective magnets 40 (not shown in FIG. 11) in the enlarged portion12′.

FIGS. 12 to 15 show embodiments of a blood pump 1, in which either thefirst bearing 10 or the second bearing 20 is supported by at least onespring. It will be appreciated that the embodiments of FIG. 12 and atleast one of FIGS. 13 to 15 could be combined in a single embodiment. Asshown in FIG. 12, a spring 42, such as a coil spring, is provided tosupport the shaft 21′, which is substantially similar to the secondshaft 21 described in connection with FIG. 7 except that it is axiallymovable and shorter to provide room for the spring 42. A sealing ring 45is provided to prevent blood from entering the motor assembly. Thespring 42 is relatively weak, in particular the spring force is lessthan a load that would act on the second bearing 20 without the spring42 and without the first bearing 10. Thus, the load on the secondbearing 20 is limited to the amount of the spring force of the spring42. The remainder of the load is supported by the first bearing 10.

Alternatively, or in addition, as shown in FIG. 13, the first bearing10, in particular its static part, may be spring supported. In thisembodiment, the supporting structure 15 is separated from the pumpcasing 2 and supported by springs 43, such as coil springs, retained bya ledge 44. The spring force of the springs 43 acts in a direction awayfrom the second bearing 20 in order to relieve the second bearing 20.The same function can be achieved by a flexible ring 46, such as apolymer O-ring, as shown in FIG. 14 instead of the springs 43.Alternatively, or in addition, the supporting structure 15 may be madeof a flexible, resilient or elastic material to provide the springfunction as shown in FIG. 15. Likewise, a spring arrangement could bedisposed in the first shaft 14 to relieve the second bearing 20.

1.-15. (canceled)
 16. A blood pump, comprising: a pump casing having ablood flow inlet and a blood flow outlet connected by a passage, and animpeller arranged in said pump casing so as to be rotatable about anaxis of rotation, the impeller being provided with blades sized andshaped for conveying blood along the passage from the blood flow inletto the blood flow outlet, the impeller being rotatably supported in thepump casing by a first bearing at a first axial end of the impeller anda second bearing axially spaced apart from the first bearing, whereinthe first bearing comprises a projection extending along the axis ofrotation and connected to one of the impeller and the pump casing and acavity in the other one of the impeller and the pump casing, theprojection comprising an enlarged portion that engages the cavity suchthat the first bearing and the second bearing are arranged to bear axialforces in the same axial direction, and wherein the second bearing is acontact-type bearing comprising a bearing surface of the impeller facinga bearing surface of the pump casing.
 17. The blood pump of claim 16,wherein the second bearing is a pivot bearing.
 18. The blood pump ofclaim 16, further comprising a shaft extending along and rotatable aboutthe axis of rotation and having the impeller mounted thereon, the shafthaving a first end portion forming part of the first bearing and asecond end portion forming part of the second bearing.
 19. The bloodpump of claim 18, wherein the first end portion of the shaft comprisesthe enlarged portion.
 20. The blood pump of claim 19, wherein the shafthas an outer diameter that is substantially equal to an outer diameterof the enlarged portion, wherein the projection forms a neck arrangedbetween the enlarged portion and a remainder of the shaft.
 21. The bloodpump of claim 18, wherein the second end portion of the shaft comprisesa bearing surface of the second bearing, said bearing surface beingconcave.
 22. The blood pump of claim 16, wherein a wall of the cavitycomprises at least two sections separated by a gap, the gap being influid connection with the passage and allowing blood to enter thecavity.
 23. The blood pump of claim 16, wherein at least one of thebearing surfaces of at least one of the first and second bearings issupported by at least one spring, wherein the at least one spring isarranged to bear axial forces in an axial direction from the secondbearing towards the first bearing.